1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus for recording information on an optical recording medium or reproducing information from the medium and, more particularly, to an opto-magnetic recording and reproducing apparatus using an opto-magnetic recording medium such as an opto-magnetic disk.
2. Description of the Related Art
Conventionally, an optical head for an optical information recording and reproducing apparatus using an opto-magnetic system is ordinarily arranged in such a manner that a light beam from a light source travels to a recording medium via a polarization beam splitter and a part of the light beam reflected by the medium diverges to travel to a photo-detector when the reflected light beam again passes through the polarization beam splitter.
In this arrangement, it is necessary for the polarization beam splitter to lead a maximum quantity of light to the medium in a forward optical path and to provide a maximum amount of a Kerr component in the divergent beam to the detector in a backward optical path. Ordinarily, in such a situation, the polarization beam splitter is disposed in a parallel beam. This is because the polarized light separating film of the polarization beam splitter ordinarily has an incident angle dependency in its characteristics. That is, the polarized light transmittance and reflectance and phase differences produced between polarized light components vary with respect to the angle of incidence of the light beam upon the polarized light separating film. There is a need to make these characteristics uniform in a cross section of the beam. Then, a need for disposing the polarization beam splitter in a parallel beam arises.
For the above-mentioned head for the opto-magnetic disk apparatus, therefore, an optical system must be formed in which a divergent beam from the light source is temporarily formed into a parallel beam by a collimator lens, and in which the parallel beam is thereafter converged on the recording medium by an objective. These two lenses, i.e., the objective and collimator lenses of the optical system, are indispensable if they are used to form an infinite. imaging system.
These kinds of apparatuses are now being developed in keen competition with magnetic disk apparatuses, and it is necessary for them to be smaller in size and higher in speed. Conventionally, under the condition of forming an infinite imaging system, a separate optical head is formed by utilizing the characteristic of the objective being freely drivable relative to a parallel beam, i.e., separating moving parts which need to be moved so that the weight of a moving unit is smaller.
In this art, however, it is difficult to reduce the number of lenses, and the necessary positioning accuracy of a separated unit and a fixed unit becomes higher with advancement of miniaturization. Thus,the optical head cannot be remarkably miniaturized as long as the head system is based on this art.
If the condition that the polarization beam splitter is placed in a parallel beam can be eliminated, an optical magnetic head as small as optical heads of compact disk players in the number of parts and in size can be arranged by removing the collimator lens and by forming a finite imaging system by the objective lens. Also in such a case, there is a possibility that a light and high-speed apparatus will be realized which uses a very small optical system further miniaturized.
Even in the case of forming an infinite system, if only the polarization beam splitter can be placed in the divergent beam in front of the collimator lens, the objective and collimator lenses can be brought close to each other and the collimator lens can also serve as a lens for converging the detected beam split by the polarization beam splitter, which is necessary in the conventional art. Thus, means for miniaturization are increased if it is not necessary to place the polarization beam splitter in a parallel beam.
FIG. 1 shows a system disclosed in Japanese Patent Laid-Open Publication No. 234173/1993 as an example of an invention provided from such a view point. As shown in FIG. 1, a divergent light beam from a semiconductor laser device 1 passes through a polarization beam splitter 26 and is then changed into a parallel beam by a collimator lens 14. This parallel beam is converged on an opto-magnetic disk 6 by an objective 15. A detected light beam is reflected by the disk, passes through the objective and the collimator lens, and is reflected by the polarization beam splitter 26, thereby being deflected in a direction different from that of the light source. The detected light beam is again reflected by the next reflecting means 27 to reach photo-detectors 10 for opto-magnetic detection and servo detection by traveling through a 1/2 wavelength plate 8, a cylindrical lens 28 and a beam splitter 29.
The beam splitter 26 and the reflecting means 27 have films having substantially the same characteristics. The detected light beam is reflected by the beam splitter 26 to have a phase difference and is then reflected by the reflecting means 27 having the same characteristics so that the incident angles of rays incident upon the reflecting means 27 are in an inverted relationship with the incident angles of rays incident upon the beam splitter 26, thereby canceling the phase difference.
The above-mentioned Japanese Patent Laid-Open Publication No. 234173/1993 points out a problem that, if a polarization beam splitter having certain ideal transmittance and reflectance characteristics, e.g., Tp=85% and Rs=100%, is modified to reduce phase differences of p- and s-polarized light, the transmittance and reflectance characteristics deteriorate so that Tp=60% and Rs=70%. According to the art of this publication, the polarization beam splitter 26 is allowed to create phase differences of p- and s-polarized light in the detected light beam reflected by it, so that the ideal transmittance and reflectance characteristics of Tp=85% and Rs=100% are maintained. Then, the reflecting means 27 having substantially the same characteristics as the polarization beam splitter 26 is placed in the optical path of the detected light beam reflected by the polarization beam splitter 26 to cancel the phase differences of p- and s-polarized light created in the detected light beam.
[Problem 1]
In general, a phase difference created in a transmitted or reflected light beam by a polarization beam splitter has an incident angle dependency due to the fact that the polarized light separating film is ordinarily a laminated film of a dielectric having characteristics determined according to the relationship between the refractive indexes, the thicknesses of layers of the film and the wavelength of the incident light beam. Ordinarily, each layer of the dielectric multilayer film is formed so as to satisfy the condition that its thickness is uniform within the range of manufacturing error in an effective diametrical area.
For example, if a ray is incident upon the film at a certain incident angle and travels in the film at a refractive angle .theta., the film designed to realize a certain characteristic is formed, to put it simply, by setting its thickness to the desired value with respect to the ray traveling at the refractive angle .theta., i.e., a value obtained by multiplying the desired distance for the ray by cos.theta.. Accordingly, if the incident light beam has an incident angle constant in the entire beam area (that is, in the case of a parallel beam), a phase difference is created uniformly through the light beam area. However, with respect to a ray incident at an angle different from the predetermined incident angle, the effective film thickness is changed according to the incident angle of the ray. As a result, the overall characteristics of the polarized light separating film differ from the desired characteristics.
In the above-described example of the conventional art, the divergent light beam in the forward optical path from the light source to the medium via the polarization beam splitter also undergoes phase modulation of the film of the polarization beam splitter. Ordinarily, in this kind of apparatus, the light source is a semiconductor laser device, the light beam in the forward optical path has only a linearly-polarized component, and the polarization beam splitter receives only a p-polarized component. As mentioned above, the phase modulation effect on rays depends on the incident angle and is not uniform in a cross section of the light beam. Even if the light beam has only a p-polarized component, it has local phase advancement and lag according to the incident angle. That is, such phase modulation causes a wavefront aberration of the light beam in the forward path.
Therefore, while a phase difference can be allowed as a reflection phase difference of the detected light beam reflected by the polarization beam splitter, it is necessary to reduce the incident angle dependency of phase variation with respect to the p-polarized component of the transmitted light beam passing through the polarization beam splitter.
The first problem to be solved by the present invention is summarized below. While importance has been set on canceling or reducing a phase difference created in the detected light beam, it is necessary to recognize the influence of the phase characteristics of the film upon the light beam in the forward path causing a wavefront aberration. To reduce this aberration, it is necessary to reduce the incident angle dependency of the phase through the film in the forward optical path.
[Problem 2]
The transmittance and reflectance also have incident angle dependencies for the same reason as in the above, and these dependencies must be considered. To maintain a high efficiency of utilization of the quantity of light from the light source, it is necessary to realize characteristics of the polarization beam splitter, e.g., Tp=85% and Rs=100%, as stated in the specification of the above-described example of the conventional art. These characteristics must be achieved with respect to the incident angle of all rays in the convergent light bream. The specification of the conventional art includes a description of the effect that, if the beam splitter is designed to reduce the phase difference, the transmittance and reflectance characteristics change so that the utilization efficiency is reduced, but it includes no description of the incident angle dependency of the transmittance and reflectance.
If the film of the polarization beam splitter is simply designed to set the desired transmittance and reflectance with respect to a principal ray without particularly considering the incident angle dependency, then certain distributions of the transmittance and reflectance occur in a direction along the incidence surface of the polarization beam splitter film. For example, in a case where a principal ray is incident on the film at 45.degree., the transmittance and reflectance are non-uniform from a point at which a marginal ray is incident at an angle of 45.degree.+.alpha. to an opposite point of an incident angle of 45.degree.-.alpha., and may have asymmetric distributions such as those shown in FIG. 2. In such a case, the quantity of light incident upon the objective in the forward optical path has an asymmetric distribution about the optical axis, so that the light spot on the medium is also asymmetric.
Another case is possible in which the transmittance has a distribution such as to be lower with respect to marginal rays at opposite points of 45.degree..+-..alpha.. In this case, the diameter of a substantial area of the light beam in the corresponding cross section is smaller, that is, the effective numerical aperture (NA) is smaller and the light spot is larger.
Thus, the light spot on the recording medium is influenced to cause a hindrance to recording and reproduction. Therefore, it is particularly important to maintain certain degrees of symmetry and uniformity of the quantity of light in the forward optical path. However, if the film is designed so that the transmittance of the p-polarized component is uniform at an intermediate value of about 85% without an angle dependency, the problem of an increase in the total number of film layers and other problems are encountered.
Further, a loss in the quantity of light is also caused by the influence of an asymmetry of the reflectance due to an incident angle dependency when the detected light beam reflected by the recording medium and returned to the polarization beam splitter is reflected by the polarization beam splitter, although this influence is not as serious as that of the forward optical path.
The second problem to be solved by the present invention is summarized below. It is necessary to recognize that the angle dependency of the polarized light separating transmission and reflection characteristics of the polarization beam splitter largely affects the shape of the detection spot on the recording medium and the loss in the quantity of light of the detected light beam and, therefore, to limit the incident angle dependency of the polarized light separating transmission and reflection characteristics of the film.
[Problem 3]
When the light beam reaches the reflecting means 27 of the conventional art, further 85% of the p-polarized component of the light beam passes through the reflecting means 27 because the films of the polarization beam splitter 26 and the reflecting means 27 in the conventional art have essentially the same characteristics. Therefore, the absolute value of the quantity of light reaching the detectors 10 is not large enough to ensure the desired detection performance against noise. Only 2.25% of the p-polarized component of the detected light beam can reach the detectors 10.
Also, it is necessary for the films having the same characteristics to have the same glass-film structure. Therefore, the reflecting means 27 must be backed by the same film-sandwiching as that for the beam splitter 26.
The third problem to be solved by the present invention is summarized below. In the conventional art, unnecessary transmission occurs due to the polarized light separating transmission and reflection characteristics of the reflecting means 27 to influence the loss in the quantity of light of the detected light beam. It is necessary to recognize that the angle dependency of the polarized light separating transmission and reflection characteristics of the reflecting means 27 also influences the loss in the quantity of peripheral light. To reduce this influence, it is necessary to optimize the polarized light separating transmission and reflection characteristics of the film and the incident angle dependency of the same. Further, to form the reflecting means 27 so that its characteristics are equal to those of the polarization beam splitter 26, additional manufacturing steps for providing a film-sandwiching glass part and attaching the glass part to the back surface of the film are required.